Apparatus and method of superconducting magnet cooling

ABSTRACT

A superconducting magnet assembly and method of cooling a superconducting magnet assembly includes thermally connecting a pulsating heat pipe to the superconducting magnet assembly and adding a liquid cryogen to the pulsating heat pipe. The superconducting magnet assembly also includes a coil former, at least one superconducting solenoid magnet comprising at least one superconducting winding wrapped about the coil former and configured to generate a magnetic field, and at least one pulsating heat pipe thermally connected to the at least one superconducting solenoid magnet. The present invention has been described in terms of specific embodiment(s), and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related in some aspects to commonly owned U.S. application serial number (to be provided), entitled “APPARATUS AND METHOD FOR COOLING A SUPERCONDUCTING MAGNETIC ASSEMBLY”, assigned attorney docket number 226913-1, filed concurrently herewith, the entire contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to the cooling of superconducting magnets and more particularly to a superconducting magnet assembly and a process for cooling the superconducting magnet assembly.

Various systems employ superconducting magnets to generate a strong, uniform magnetic field within which a subject, or as in the case of an MRI system a patient, is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Systems that use such coils include MRI systems, spectroscopy systems, magnetic energy storage systems, and superconducting generators.

A superconducting magnet is typically immersed in a cryostat that includes a thermal shield and a vacuum vessel that insulate the magnet from the environment during operation. The superconducting magnet also has a coil support structure to support the coil winding, embedded in a cold mass and a helium vessel for cooling. The helium vessel is a pressure vessel located within the vacuum vessel for thermal isolation and typically contains liquid helium to provide cooling for the superconducting magnet to maintain a temperature of around 4.2 Kelvin for superconducting operation.

A significant cost for any system that employs superconducting magnets is for the provision of helium cryogen. Helium, or a similar cryogen (e.g., neon), is needed both for initial start-up and operation of the superconducting magnet, and for keeping the magnet in a pool-boiling state. While thermodynamically efficient, a full helium bath, when employed in a superconducting magnet assembly, requires a relatively large volume of helium on the order of approximately 1,500 to 2,000 liters. Helium is expensive per unit volume, is not always readily obtainable, and its cost is increasing.

Accordingly, there is an ongoing need for reducing both the manufacturing and operating cost and/or simplifying the design of superconducting magnet systems.

BRIEF DESCRIPTION

The present invention overcomes at least some of the aforementioned drawbacks by providing a superconducting magnet assembly, and a method of cooling a superconducting magnet assembly, that reduces the amount of cryogen needed for cooling the superconducting magnet. More specifically, the present invention is directed to provide a superconducting magnet assembly that employs a pulsating heat pipe thereby simplifying the design and assembly and requires only a very small quantity of cryogen.

Therefore, in accordance with one aspect of the invention, a method of cooling a superconducting magnet assembly includes thermally connecting a pulsating heat pipe to the superconducting magnet assembly, and adding a liquid cryogen to the pulsating heat pipe.

In accordance with another aspect of the invention, a superconducting magnet assembly includes: a coil former; at least one superconducting solenoid magnet comprising at least one superconducting winding wrapped about the coil former and configured to generate a magnetic field; and at least one two-phase heat transfer device thermally connected to the at least one superconducting solenoid magnet.

Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one embodiment presently contemplated for carrying out the invention.

FIG. 1 is a schematic diagram of a superconducting magnetic assembly incorporating aspects of the present invention.

FIG. 2 is a perspective view of a superconducting magnet assembly with a partial cut-away according to an embodiment of the present invention.

FIG. 3 is a top cross-sectional view of a portion of tubing of the superconducting magnet assembly of FIG. 2.

FIG. 4 is a side cross-sectional view of a condenser region of the superconducting magnet assembly of FIG. 2.

FIG. 5 is a side cross-sectional view of a coil former of the superconducting magnet assembly of FIG. 2.

DETAILED DESCRIPTION

Aspects of the present invention have been shown to offer advantages over previous methodologies of cooling superconducting magnets. The apparatus and method require no mechanical moving parts (e.g., no pumps, no external pressure fed supply system, no refrigeration “coldbox” for cryogen supply, no helium can-fed flow, etc.) in cooling a superconducting magnet. The cooling method is orientation-independent, which aids in the design and ultimate physical volume and footprint of the superconducting magnet assembly. Design integration of tubing into the existing superconducting magnet geometry is simplified. Further, any hot spots that may arise can immediately be cured by the pulsating slug flow provided by the pulsating heat pipe employed in aspects of the present invention. The design offers other advantages in that it does not require costly helium bath cooling, nor is any cryogen lost during a quench. The proposed capillary tubing can withstand high pressures of several 100 bars. As a result, no helium reservoir is required. The absence of a helium reservoir and its cryogenic housing is a major simplification in that all safety aspects that need to be adhered to are greatly reduced. Further, the heat loads to the magnet are greatly reduced and minimized due to the absence of the traditional vertical neck or penetration geometry, typical for a standard helium vessel. Penetrations or access ports are known to causes high heat leaks to a magnet. A desirable result of the improved design is that the usable room temperature bore width of the magnet can be increased. Furthermore, the suspension system required to keep the helium vessel floating in the vacuum vessel is further simplified and the heat leak reduced. Ultimately, the cooling system of the superconducting magnet system is greatly simplified.

Referring to FIGS. 1 and 2, the major components of a superconducting magnetic assembly 100 incorporating aspects of the present invention are shown. The superconducting magnetic assembly 100 may include a coil former 102 (FIG. 2) and a superconducting winding wrapped about the coil former 102 thus comprising a superconducting magnet configured so as to generate a magnetic field. A two-phase heat transfer device (e.g., pulsating heat pipe) 10 is thermally connected to the superconducting magnet. The pulsating heat pipe 10 is designed and configured so as to provide adequate cooling of the superconducting magnet assembly 100. The pulsating heat pipe 10 may comprise any now known or later developed pulsating heat pipe system 10. For example, the pulsating heat pipe 10 may comprise tubing 40, a condenser portion 30, and an evaporator portion 20.

The superconducting magnet assembly 100 may include a solenoid wound on one or several magnet former pockets. The magnet, or coil, former may be made of any suitable material, such as glass fiber reinforce plastic, composite material, metal (e.g., steel, aluminum, magnesium, etc.), ceramics, or combinations thereof.

The pulsating heat pipe 10 (e.g., tubing 40) is partially filled with a liquid cryogen by first cooling the magnet close to the thermal shield temperature. The tubing 40 is then charged with high-pressure gas from room temperature. The magnet cryocooler convectively liquefies the gas entering the tubing 40 thereby reducing the gas pressure significantly in the tubing 40 and producing droplets of cryogens. In order for the tubing 40 and other system components to act as a pulsating heat pipe 10, less than the entirety of the tubing 40 volume may be filled with the cryogen. Thus, while a portion of the tubing 40 volume is filled with the liquid cryogen 44, the remaining portion has still vapor cryogen (e.g., cryogen bubbles 46) therein. The liquid cryogen may comprise one of helium 4, helium 3, hydrogen, neon, nitrogen, oxygen, argon, krypton, and combinations thereof. Other suitable cryogens may be used in other embodiments depending on the type of superconductor used for the magnet. It has been discovered that various mixtures of liquid 44 and vapor 46 cryogen within the tubing 40 portion of the pulsating heat pipe 10 work in dissipating heat generated from the superconducting magnet assembly 100. For example, in certain embodiments the ratio of liquid cryogen to the total volume of the tubing 40 can be in a range from about 10% to about 90%. Similarly, in other embodiments the ratio of liquid cryogen to the total volume of the tubing 40 can be in a range from about 30% to about 70%. The balance of the total volume of tubing 40 (i.e., portion not filled with liquid cryogen) is filled with vapor cryogen (e.g., cryogen bubbles 46). Thus, vapor cryogen may fill a percentage of the total volume of tubing 40 in a range from about 90% to about 10%. In other embodiments the ratio of vapor cryogen to the total volume of tubing 40 can be in a range from about 70% to about 30%. In this manner a mixture of liquid cryogen and vapor cryogen work to cool the superconducting magnet assembly 100.

Various configurations of a condenser portion 30 may be used. The condenser portion 30 may be a cross flow heat exchanger, as depicted in FIG. 2 having finned heat exchange fins 32 thereon. The heat exchanger may be made of copper or other suitable material. There may be direct thermal contact between the condenser portion 30 and the tubing 40. FIG. 4 shows a close-up view of the interface between a closed loop pulsating heat pipe 10 and a condenser 30 having finds 32 thereon. Clearly, other geometries and orientations may be employed for the condenser portion 30 that adequately provide cooling of the superconducting magnet assembly 100.

Similarly, various configurations of an evaporator portion 20 may be used. The evaporator portion 20 may, as shown in FIG. 2, only require a simple epoxy coil former. As a result, there is no need to increase the thermal conductivity of the former using fillers used in prior art designs. While considerable efforts have been extended in the past 30 years to develop epoxy materials, composites, and the like, with high-thermal conductivity fillers they have only been met with limited success to increase the heat spreading performance of the coil former. More recently, heat sinking material may be embedded in the magnet former. While all of these approaches add cost to the system more importantly they increase the risk of inducing stress on the magnet former. The magnet former can crack, and the crack(s) can further propagate upon rapid cooling down or warming up of the magnet. It is not unheard of for superconducting magnets to fail due to epoxy cracking. Under aspects of the present invention, the need for any heat spreading mechanism is avoided.

While the tubing 40 in FIGS. 1 and 2 is shown as a separate serpentine, closed system pattern, the tubing 40 in embodiments of the present invention can be arranged in a variety of configurations. The tubing 40 may be arranged in a closed loop or an open loop system. The tubing 40 is shown in FIG. 2 as two separate closed loop systems. A first pulsating heat pipe 10 may be configured at or near the periphery of the coil former 20 while the second pulsating heat pipe 10 is configured at or near the interior bore of the coil former 20. The tubing 40 may be any quantity of individual tubes 40 (i.e., capillaries) ranging from a single tube 40 to a near infinite quantity of separate tubes 40. The geometry of each tube 40 may also vary from a straight tube 40 extending from evaporator to condenser to tubing 40 with a plurality of turns 42 therein. The tubing 40 may be arranged in an organized, serpentine, and horizontal (or slightly inclined) pattern as shown in FIGS. 1 and 2. Contrastingly, the tubing 40 may be arranged in a non-repeating, asymmetric, and/or non-planar arrangement while still providing adequate cooling means of the superconducting magnet assembly 100. An advantage of aspects of the present invention is that the geometry and arrangement of the tubing 40 may be entirely independent of gravity and orientation. In other words, gravity and the orientation of the pulsating heat pipe 10 and tubing 40 is not known to substantially affect the flow and the cooling performance of the liquid and vapor cryogen in the pulsating heat pipe 10. The tubing 40 may, for example, be substantially horizontal, substantially vertical, or combinations thereof. In any event, the tubing 40 geometry may be adapted and arranged to fit and match with the bobbin 102 or other elements of the superconducting magnet assembly 100 to which the tubing 40 is thermally connected. This allows for increased flexibility of the manufacturing size and arrangement of the entire superconducting magnet assembly 100 in that the cooling mechanism would not typically require additional and/or significant design space.

The end section of the superconducting magnet assembly 100 is shown in FIG. 5. The plurality of tubing 40 is shown substantially surrounding the outer periphery of the coil former. Similarly, a plurality of tubing 40 is substantially surrounding the inner core of the coil former. In this manner the pulsating heat pipe(s) 10 are able to effectively cool the coil former and the superconducting magnet assembly 100, as a whole. While FIG. 5 shows the tubing 40 running substantially longitudinally along the axis of the coil former, it should be apparent that other configurations of tubing 40 are possible under aspects of the invention. For example, with the coil former shown, the tubing 40 may run around the periphery of the coil former (e.g., normal to the direction the tubing 40 is running in FIG. 5).

The total volume of all tubing 40 for the pulsating heat pipe 10 may be in a range of approximately 10 ml to approximately 2 liters depending on the superconducting magnet assembly 100 size and application. The tubing 40 may be made of any suitable material such as copper and its alloys, aluminum and its alloys, steel, and the like. Alternatively, the tubing 40 may be an internally clad capillary or formed tubing 40. The inside diameter of the tubing 40 may be in a range of approximately 1 mm to approximately 8 mm. Similarly, the diameter of the tubing 40 need not be uniform over the entire length of the tubing 40 but may vary over its length. For example, the diameter of the tubing 40 in the condensing portion may be narrower than in other portions of the pulsating heat pipe 10 in order to slow down flow velocity of the cryogen. Similarly, the cross-section of the tubing 40 may be other shapes, such as square, rectangular, oval-shaped, and the like.

A method of cooling a superconducting magnet assembly 100 includes thermally connecting a pulsating heat pipe 10 to a superconducting magnet assembly 100. The tubing 40 is first evacuated and then partially filled with a cryogen under high pressure as discussed herein. Partially filling comprises filling in a range from about 10% to about 90% of the total volume of the tubing 40 with a liquid cryogen 44. The remaining volume of the tubing 40 may comprise vapor cryogen (i.e., bubbles 46). In this manner, the working fluid (i.e., cryogen) naturally distributes over the length of tubing 40 into distinct liquid plugs 44 and vapor bubbles 46. In this manner various tube sections of the tubing 40 in the pulsating heat pipe 10 have different volumetric fluid/vapor distribution. As the pulsating heat pipe 10 operates, each tubing 40 section at the evaporator portion 20 is heated by virtue of its adjacency to the superconducting magnet assembly 100. Similarly, each tubing 40 section at the condenser portion 30 is cooled. As a result, vapor cryogen bubbles 46 are generated and/or grow in the evaporator region and collapse and/or shrink in the condenser portion 30. This change in vapor bubble 46 size concomitantly causes liquid cryogen 44 transport due to the bubble pumping action, ultimately resulting in sensible heat transfer within the pulsating heat pipe 10. Thermally induced self-excited oscillations commence.

The cooling apparatus 10 may be designed in accordance with aspects of the present invention so that the vapor bubbles 46 have an opportunity to lose their entire latent heat in the condenser portion 30 thereby collapsing in size. This requires that the residence time of the vapor bubble 46 in the condenser portion 30 should be sufficient for complete condensation of the vapor bubble 46. Each vapor bubble 46 carries a relatively small amount of enthalpy, more and more vapor bubbles 46 should get an opportunity to loose their latent heat in the condenser portion 30 so that their integrated effect exceeds the frictional disadvantages that may be caused by their presence in the tubing 40. There should be enough liquid plugs 44 in the pulsating heat pipe 10 for substantial sensible heat transfer.

An aspect of the present invention includes a goal of low thermal resistance and good heat transfer from the pulsating heat pipe 10. It has been discovered under aspects of the present invention that with increasing heat loads on the evaporator portion 20 that the efficiency of the pulsating heat pipe 10 increases accordingly. Typically, with a generic (i.e., non-cryogen) pulsating heat pipe a 30% fill charge (i.e., 30% of total volume filled with non-cryogen liquid coolant) is an optimal fill charge for efficiency purposes.

While exemplary embodiments of the present invention are able to cool superconducting magnet assemblies to around 4.2 Kelvin for superconducting operation, other operating temperatures besides 4.2 Kelvin may be employed without departing from the scope of the invention. For example, superconductors with higher transition temperatures (e.g., of HTS type or MgB2-type) can be cooled under aspects of the present invention.

While the embodiments illustrated and described herein may be used with a superconducting magnet assembly 100 that is part of a magnetic resonance imaging (MRI) system, other superconducting magnet systems may employ aspects of the present invention without departing from the scope of the invention. For example, the cooling apparatus and method of cooling may be used with other superconducting magnets such as nuclear magnetic resonance spectroscopy systems, magnetic energy storage systems, superconducting generators, superconducting fault current limiters, superconducting particle accelerators, magnetic separation systems, transportation systems, superconducting cables, transformers, superconducting supercomputers, space and aeronautics applications, and the like.

Therefore, according to one embodiment of the present invention, a method of cooling a superconducting magnet assembly includes thermally connecting a pulsating heat pipe to the superconducting magnet assembly, and adding a liquid cryogen to the pulsating heat pipe.

According to another embodiment of the present invention, a superconducting magnet assembly superconducting magnet assembly includes: a coil former; at least one superconducting solenoid magnet comprising at least one superconducting winding wrapped about the coil former and configured to generate a magnetic field; and at least one two-phase heat transfer device thermally connected to the at least one superconducting solenoid magnet.

The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims. 

1. A method of cooling a superconducting magnet assembly comprising: thermally connecting at least one pulsating heat pipe to the superconducting magnet assembly; and adding a liquid cryogen to the at least one pulsating heat pipe.
 2. The method of claim 1 wherein the liquid cryogen comprises one of helium 4, helium 3, hydrogen, neon, nitrogen, oxygen, argon, krypton, and combinations thereof.
 3. The method of claim 1 wherein the liquid cryogen fills a percentage of a total volume of the at least one pulsating heat pipe in a range from about 10% to about 90%.
 4. The method of claim 3 further wherein the liquid cryogen fills a percentage of a total volume of the at least one pulsating heat pipe in a range from about 30% to about 70%.
 5. The method of claim 1, wherein the at least one pulsating heat pipe is a closed system.
 6. The method of claim 1, wherein the superconducting magnet assembly is configured for use with one of a nuclear magnetic resonance spectroscopy system, a magnetic energy storage system, a superconducting generators, a superconducting fault current limiter, a superconducting particle accelerator, a magnetic separation system, a transportation systems, a superconducting cable, a transformer, and a superconducting supercomputer.
 7. The method of claim 1, wherein the at least one pulsating heat pipe is an open system.
 8. The method of claim 1, wherein the at least one pulsating heat pipe comprises tubing and a condenser.
 9. The method of claim 1, wherein the at least one pulsating heat pipe is embedded in an epoxy structure of the superconducting magnet assembly.
 10. A superconducting magnet assembly comprising: a coil former; at least one superconducting solenoid magnet comprising at least one superconducting winding wrapped about the coil former and configured to generate a magnetic field; and at least one two-phase heat transfer device thermally connected to the at least one superconducting solenoid magnet.
 11. The superconducting magnet assembly of claim 10, wherein the coil former is comprised of a thermally conductive material.
 12. The superconducting magnet assembly of claim 10 further comprising a cryogen in the at least one two-phase heat transfer device.
 13. The superconducting magnet assembly of claim 12 wherein the cryogen comprises one of helium 4, helium 3, hydrogen, neon, nitrogen, oxygen, argon, krypton, and combinations thereof.
 14. The superconducting magnet assembly of claim 12 wherein a liquid portion of the cryogen fills a percentage of a total volume of the at least one two-phase heat transfer device in a range from about 10% to about 90%.
 15. The superconducting magnet assembly of claim 14 further wherein a liquid portion of the cryogen fills a percentage of a total volume of the at least one two-phase heat transfer device in a range from about 30% to about 70%.
 16. The superconducting magnet assembly of claim 10, the at least one two-phase heat transfer device comprises at least one pulsating heat pipe comprising tubing and a condenser.
 17. The superconducting magnet assembly of claim 16 wherein an inside diameter of the tubing is in a range of approximately 1 mm to approximately 8 mm.
 18. The superconducting magnet assembly of claim 16 wherein the at least one pulsating heat pipe is a closed system.
 19. The superconducting magnet assembly of claim 16 wherein the at least one pulsating heat pipe is an open system.
 20. The superconducting magnet assembly of claim 16 wherein the at least one pulsating heat pipe is configured in a serpentine pattern.
 21. The superconducting magnet assembly of claim 10, wherein a total volume of cryogen in the at least one two-phase heat transfer device is in a range of approximately 10 ml liter to approximately 2 liters.
 22. The superconducting magnet assembly of claim 10, wherein a flow geometry of the at least one two-phase heat transfer device is one of substantially horizontal, substantially vertical, and combinations thereof.
 23. The superconducting magnet assembly of claim 10, wherein a sizing of the at least one two-phase heat transfer device is dependent upon a heat load imposed by the at least one superconducting magnet.
 24. The superconducting magnet assembly of claim 16, wherein a configuration of the at least one pulsating heat pipe is adapted to a geometry of the coil former. 